Improving outcomes for low-income fathers and their children is an important goal, but there is limited evidence about which approaches are most effective. Building Bridges and Bonds (B3) is a rigorous evaluation study of responsible fatherhood programs designed to answer multiple practice-relevant research questions about individual program components. The broad goals are to build substantive knowledge about the effective practices and components of responsible fatherhood programs, and identify strategies to improve program effectiveness. The specific research questions will be determined in the first year of the project, with input from fatherhood research and practice experts on the highest priority questions in the field. Potential directions include rigorously testing implementation features such as recruitment and retention, sequencing of services, building coalitions with community partners, testing the efficacy of program components (e.g., innovative employment services, parenting and coparenting skills development, or couple/relationship education), etc. Six fatherhood programs will be involved in impact and process studies. All impact tests will be conducted through efficient random assignment designs, such as factorial and multi-arm trials, and will focus on short-term impacts. Data sources will include the program's management information system, participant surveys at baseline and 6-month follow-up, and administrative records. The process studies will highlight operational lessons for the field and provide context for understanding the impact results. They will include both quantitative and qualitative data, which will be collected through observation, surveys, interviews, and focus groups with program staff and participating fathers.